Rollup merge of #94327 - Mark-Simulacrum:avoid-macro-sp, r=petrochenkov

Avoid emitting full macro body into JSON errors

While investigating https://github.com/rust-lang/rust/issues/94322, it was noted that currently the JSON diagnostics for macro backtraces include the full def_site span -- the whole macro body.

It seems like this shouldn't be necessary, so this PR adjusts the span to just be the "guessed head", typically the macro name. It doesn't look like we keep enough information to synthesize a nicer span here at this time.

Atop #92123, this reduces output for the src/test/ui/suggestions/missing-lifetime-specifier.rs test from 660 KB to 156 KB locally.